There has recently been considerable research into organic thin-film light emitting devices comprising an emissive layer containing an organic fluorescent body existing between an anode and a cathode, which emit light when electrons injected from a cathode and holes injected from an anode recombine within an organic fluorescent body interposed between the cathode and the anode. Such light emitting devices are a focus of attention on account of their characteristics of thin shape, high luminance at a low driving voltage and polychromic light emission based on suitable selection of the fluorescent materials.
Numerous research organizations have been carrying out such research since C. W. Tang and co-workers at Kodak first described the fact that an organic thin-film light emitting device emits light of high luminance. A typical organic thin-film light emitting element construction proposed by the Kodak research group is that in which there are provided, in turn, on an ITO glass substrate, a hole transporting diamine compound, tris(8-hydroxyquinolate)aluminum(III) as the emissive layer and Mg:Ag as the cathode. A 1,000 cd/m2 green light emission is possible at a driving voltage of about 10 V (refer to Non-Patent Document 1).
One of largest problems associated with organic thin-film light emitting devices is to improve durability and luminance efficiency of the device. There is known a method of forming an emissive layer by doping a host material with several % of a dopant material as means for obtaining a high-efficiency light emitting device. High carrier mobility and uniform film forming properties are required for the host material, while high fluorescence quantum yield and uniform dispersibility are required for the dopant material. For example, there is disclosed a technique of using a styrylamine derivative, a perylene derivative and a pyrene derivative as a blue emissive material. There is also disclosed a technique of using a combination of a stilbene-based compound, a quinoline derivative and a quinacridon derivative as a green emissive material. There is also disclosed a technique of using a combination of an aminostyryl derivative, a cumarin derivative and a dicyanomethylenepyran derivative as a red emissive material. There is also disclosed a technique of using a pyrromethene-based metal complex as a dopant material (refer to Patent Document 1) and a technique of using a naphthacene derivative in combination with a fluoranthene derivative (refer to Patent Document 2). However, all of the resultant devices had insufficient chromatic purity and durability.
[Non-Patent Document 1]
    “Applied Physics Letters”, (USA), 1987, Vol. 51, No. 12, pp. 913-915[Patent Document 1]    Japanese Patent No. 3,389,888[Patent Document 2]    Japanese Unexamined Patent Publication (Kokai) No. 2003-338377